1. Field of the Invention
The present invention relates to a plate-shaped sensor element having a solid electrolyte layer, an insulating layer formed on the solid electrolyte layer, and electrode pads formed on the insulating layer; to a gas sensor including the sensor element; and to a method of manufacturing the sensor element.
2. Description of the Related Art
Conventionally, a plate-shaped sensor element is known which has a solid electrolyte layer; an insulating layer covering the solid electrolyte layer; and electrode pads formed on the insulating layer and electrically connected to respective connection terminals (Patent Document 1, FIG. 7).
A certain sensor element has a chamfered portion which is formed by cutting off an edge of the sensor element. The chamfered portion can prevent chipping at the edge of the sensor element. Also, a chamfered portion formed at a rear end of the sensor element can facilitate assembly to another member (a contact member, a lead frame, or the like).
The electrode pads are formed on a rear-end side of the sensor element and in a region in contact with the chamfered portion.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2001-188060 (FIGS. 7 and 9)
3. Problems to be Solved by the Invention:
In the case where the electrode pads are formed in a region in contact with a chamfered portion, and the chamfered portion is formed by cutting off a portion of the solid electrolyte layer and a portion of the insulating layer, the electrode pads may be partially cut off, to thereby generate chipping pieces. The chipping pieces can electrically connect the electrode pad(s) and the solid electrolyte layer so as to form an improper conductive path or cause a short circuit.
FIG. 8 shows a state of a rear end portion of a conventional sensor element before and after chamfering. In the conventional sensor element, electrode pads are formed in a region in contact with a chamfered portion. FIG. 8 shows, in a plan view and a side view, the rear end portion of the sensor element as viewed before and after chamfering.
As shown in FIG. 8, a sensor element 101 has a solid electrolyte layer 103, an insulating layer 105, and a plurality of electrode pads 107. Before chamfering, the electrode pads 107 are formed in contact with a rear end of the sensor element 101 (see the plan view and the side view before chamfering in FIG. 8).
By partially cutting off the solid electrolyte layer 103, the insulating layer 105, and the rear end portion of the electrode pads 107, an edge between a main surface and rear-end surface of the sensor element 101 is formed into a chamfered portion 109. FIG. 8 shows a state in which the electrode pads 107 are partially cut off with a resultant generation of foreign matter 111 thereby electrically connecting each of the two electrode pads 107 and the solid electrolyte layer 103 (see the plan view and the side view after chamfering in FIG. 8).
When, as mentioned above, the chipping pieces 111 electrically connect the electrode pads 107 and the solid electrolyte layer 103 to thereby form improper conductive paths therebetween, current which is expected to flow through proper conductive paths flows through unintended conductive paths. As a result, a sensor output (in other words, sensor current which flows through the proper conductive paths) assumes an incorrect value, resulting in a reduction in detection accuracy of the sensor element. When excess current flows through the solid electrolyte layer 103 as a result of the chipping pieces 111 forming the improper conductive paths, a state (so-called blackening) in which the solid electrolyte layer 103 lacks oxygen arises, potentially resulting in damage to the sensor element.
In the case where the electrode pads 107 are formed in a region in contact with the chamfered portion 109, the distance between the electrode pads 107 and the solid electrolyte layer 103 as measured via an end face of the insulating layer 105 becomes short. Thus, chipping pieces present in a working environment of the sensor element 101 may adhere to the sensor element 101 and form an improper conductive path(s) between the electrode pad(s) 107 and the solid electrolyte layer 103.